A plastic is an organic polymer, available as a resin. These resins can be liquid or paste and can be used for embedding, coating, and adhesive bonding; or they can be molded, laminated, or formed into desired shapes, including sheet, film, or larger mass bulk shapes.
The number of basic plastic materials is large and the list is increasing. In addition, the number of variations and modifications to these basic plastic materials is also quite large. Taken together, the resultant quantity of materials available is too large to be completely understood and correctly applied by anyone other than those whose day-to-day work puts them in direct contact with a diverse selection of materials. The practice of mixing brand names, trade names, and chemical names of various plastics only makes the problem of understanding these materials more troublesome. Another variable that makes it difficult for those not versed in plastics to understand and properly design with plastics is the large number of processes by which plastics can be fabricated. Fortunately, there is an organized pattern on which an orderly presentation of these variables can be based. While there are numerous minor classifications for polymers, depending on how one wishes to categorize them, nearly all can be placed into one of two major classifications--thermosetting materials (or thermosets) and thermoplastic materials. Likewise, foams, adhesives, embedding resins, elastomers, and so on, can be subdivided into the thermoplastic and thermosetting classifications. Thermosetting plastics are cured, set, or hardened into a permanent shape.
Curing is an irreversible chemical reaction known as cross-linking, which usually occurs under heat. For some thermosetting materials, curing is initiated or completed at room temperature. Even here, however, it is often the heat of the reaction, or the exotherm, which actually cures the plastic material. Such is the case, for instance, with a room-temperature-curing epoxy or polyester compound. The cross-linking that occurs in the curing reaction is brought about by the linking of atoms between or across two linear polymers, resulting in a three-dimensional rigid chemical structure. Although the cured part can be softened by heat, it cannot be remelted or restored to the flowable state that existed before curing. Continued heating for long times leads to degradation or decomposition.
Thermoplastics differ from thermosets in that they do not cure or set under heat as do thermosets. Thermoplastics merely soften, or melt when heated, to a flowable state, and under pressure they can be forced or transferred from a heated cavity into a cool mold. Upon cooling in a mold, thermoplastics harden and take the shape of the mold. Since thermoplastics do not cure or set, they can be remelted and then rehardened by cooling. Thermal aging, brought about by repeated exposure to the high temperatures required for melting, causes eventual degradation of the material and so limits the number of reheat cycles.
All polymers are formed by the creation of chemical linkages between relatively small molecules, or monomers, to form very large molecules, or polymers. As mentioned, if the chemical linkages form a rigid, cross-linked molecular structure, a thermosetting plastic results. If a somewhat flexible molecular structure with minimal or no cross-linking is formed, either linear or branched, a thermoplastic results.
Polymerization Reactions
Polymerization reactions may occur in a number of ways, with four common techniques being bulk, solution, suspension, and emulsion polymerization. Bulk polymerization involves the reaction of monomers or reactants among themselves, without placing them in some form of extraneous media, as is done in the other types of polymerization.
Solution polymerization is similar to bulk polymerization, except that whereas the solvent for the forming polymer in bulk polymerization is the monomer, the solvent in solution polymerization is usually a chemically inert medium. The solvents used may be complete, partial, or nonsolvents for the growing polymer chains.
Suspension polymerization normally is used only for catalyst-initiated or free radical addition polymerizations. The monomer is dispersed mechanically in a liquid, usually water, which is a nonsolvent for the monomer as well as for all sizes of polymer molecules which form during the reaction. The catalyst initiator is dissolved in the monomer, and it is preferable that it does not dissolve in the water so that it remains with the monomer. The monomer and the polymer being formed from it stay within the beads of organic material dispersed in the phase. Actually, suspension polymerization is essentially a finely divided form of bulk polymerization. The main advantage of suspension polymerization over bulk is that it allows cooling of the exothermic polymerization reaction and maintains closer control over the chain-building process. By controlling the degree of agitation, monomer-to-water ratios, and other variables it is also possible to control the particle size of the finished polymer, thus eliminating the need to reform the material into pellets from a melt, as is usually necessary with bulk polymerization.
Emulsion polymerization is a technique in which addition polymerizations are carried out in a water medium containing an emulsifier (a soap) and a water-soluble initiator. Emulsion polymerization is much more rapid than bulk or solution polymerization at the same temperatures and produces polymers with molecular weights much greater than those obtained at the same rate in bulk polymerizations.
In emulsion polymerization, the monomer diffuses into micelles, which are small spheres of soap film. Polymerization occurs within the micelles. Soap concentration, overall reaction-mass recipe, and reaction conditions can be varied to provide control of the reaction rate and yield.
The usual sequence of processing a thermoplastic is to heat the material so that it softens and flows, force the material in the desired shape through a die or in a mold, and chill the melt into its final shape. By comparison, a thermoset is typically processed by starting out with partially polymerized material, which is softened and activated by heating (either in or out of the mold), forcing it into the desired shape by pressure, and holding it at the curing temperature until final polymerization reaches the point where the part hardens and stiffens sufficiently to keep its shape when demolded.
Plastic-Fabrication Processes and Forms
There are many plastic-fabrication processes, and a wide variety of plastics can be processed by each of these processes or techniques. Fabrication processes can be broadly divided into pressure processes and pressureless or low-pressure processes. Pressureless or low-pressure processes include potting, casting, impregnating, encapsulating, and coating. Pressure processes are usually either thermoplastic-materials processes (such as injection molding, extrusion, and thermoforming) or thermosetting processes (such as compression molding, transfer molding, and laminating).
Compression Molding and Transfer Molding
Compression molding and transfer molding are the two major processes used for forming molded parts from thermosetting raw materials. The two can be carried out in the same type of molding press, but different types of molds are used. The thermosetting materials are normally molded by the compression or transfer process, but it is also possible to mold thermoplastics by these processes since the heated thermoplastics will flow to conform to the mold-cavity shape under suitable pressure. These processes are usually impractical for thermoplastic molding, however, since after the mold cavity is filled to its final shape, the heated mold would have to be cooled to solidify the thermoplastic part. Since repeated heating and cooling of this large mass of metal and the resultant long cycle time per part produced are both objectionable, injection molding is commonly used to process thermoplastics.
Compression Molding
In compression molding, the open mold is placed between the heated platens of the molding press, filled with a given quantity of molding material, and closed under pressure, causing the material to flow into the shape of the mold cavity. The actual pressure required depends on the molding material being used and the geometry of the mold. The mold is kept closed until the plastic material is suitably cured. Then the mold is opened, the part ejected, and the cycle repeated. The mold is usually made of steel with a polished or plated cavity.
The simplest form of compression molding involves the use of a separate self-contained mold or die that is designed for manual handling by the operator. It is loaded on the bench, capped, placed in the press, closed, cured, and then removed for opening under an arbor press. The same mold in most instances (and with some structural modifications) can be mounted permanently into the press and opened and closed as the press itself opens and closes. The press must have a positive up-and down movement under pressure instead of the usual gravity drop found in the standard hand press.
Transfer Molding
The molding material is first placed in a heated pot, separate from the mold cavity. The hot plastic material is then transferred under pressure from the pot through the runners into the closed cavity of the mold.
The advantage of transfer molding lies in the fact that the mold proper is closed at the time the material enters.
Parting lines that might give trouble in finishing are held to a minimum. Inserts are positioned and delicate steel parts of the mold are not subject to movement. Vertical dimensions are more stable than in straight compression. Also, delicate inserts can often be molded by transfer molding, especially with the low-pressure molding compounds.
Injection Molding
Injection molding is the most practical process for molding thermoplastic materials. The operating principle is simple, but the equipment is not.
A material with thermoplastic qualities--one that is viscous at some elevated temperature and stable at room temperature without appreciable deterioration during the cycle--is maintained in a heated reservoir. This hot, soft material is forced from the reservoir into a cool mold. The mold is opened as soon as the material has cooled enough to hold its shape on demolding. The cycle speed is determined by the rapidity with which the temperature of the material used can be reduced, which in turn depends on the thermal conductivity of that material. Acrylics are slow performers, and styrenes are among the fastest.
The machine itself is usually a horizontal cylinder whose bore determines the capacity. Within the bore is a piston which, when retracted, opens a hole in the top of the cylinder through which new material can be added to replace the charge shot into the mold. The cylinder is heated by electric bands which permit temperature variation along its length. Inside the exit end of the cylinder is a torpedo over which the hot material is forced just before coming out of the nozzle into the channels leading to the cavities. This gives the material a final churning and ensures thorough heating. The mold opens and closes automatically, and the whole cycle is controlled by timers.
Thermoset Injection Molding
Because of the chemical nature of the plastic materials, injection molding has traditionally been the primary molding method for thermoplastics, and compression and transfer molding have been the primary molding methods for thermosetting plastics. Because of the greater molding cycle speeds and lower molding costs in injection molding, thermoplastics have had a substantial molding cost advantage over thermosets. As a result, advances in equipment and in thermosetting molding compounds have resulted in a rapid transition to screw-injection, in-line molding. This has been especially prominent with phenolics, but other thermosets are also included to varying degrees. The growth in screw-injection molding of phenolics has been extremely rapid. The development of this technique allows the molder to automate further, reduce labor costs, improve quality, reduce rejects, and gain substantially overall molding cycle efficiency.
Extrusion and Pultrusion
The process of extrusion consists basically of forcing heated, melted plastic continuously through a die, which has an opening shaped to produce a desired finished cross section. Normally it is used for processing thermoplastic materials, but it can also be used for processing thermosetting materials. The main application of extrusion is the production of continuous lengths of film, sheeting, pipe, filaments, wire jacketing, and other useful forms and cross sections. After the plastic melt has been extruded through the die, the extruded material is hardened by cooling, usually by air or water.
Extruded thermosetting materials are used increasingly in wire and cable coverings. The main object here is the production of shapes, parts, and tolerances not obtainable in compression or transfer molding. Pultrusion is a special, increasingly used technique for pulling resin soaked fibers through an orifice, as it offers significant strength improvements. Any thermoset, granular molding compound can be extruded and almost any type of filler may be added to the compound. In fiber-filled compounds, the length of fiber is limited only by the cross-sectional thickness of the extruded piece.
A metered volume of molding compound is fed into the die feed zone, where it is slightly warmed. As the ram forces the compound through the die, the compound is heated gradually until it becomes semi-fluid. Before leaving the die, the extruded part is cured by controlling the time it takes to travel through a zone of increasing temperature. The cured material exits from the die at temperatures of 300 to 350.degree. F. and at variable rates.
Thermosetting Plastics
Plastic materials included in the thermosetting plastic category are alkyds, diallyl phthalates, epoxies, melamines, phenolics, polyesters, silicones, and ureas. In general, unfilled thermosetting plastics tend to be harder, more brittle, and not as tough as thermoplastics. Thus, it is common practice to add fillers to thermosetting materials. A wide variety of fillers can be used for varying product properties. For molded products, usually compression or transfer molding, mineral or cellulose fillers are often used as lower-cost, general-purpose fillers, and glass fiber fillers are often used for optimum strength or dimensional stability. It should be added that filler form and filler surface treatment can also be major variables. Thus it is important to consider fillers along with the thermosetting material, especially for molded products. Other product forms may be filled or unfilled, depending on requirements.
Alkyds
Alkyds are available in granular, rope, and putty form, some suitable for molding at relatively low pressures, and at temperatures in the range of 300 to 400.degree. F. They are formulated from polyester-type resins. Other possible monomers, aside from styrene, are diallyl phthalate and methyl methacrylate. Alkyd compounds are chemically similar to the polyester compounds but make use of higher-viscosity, or dry, monomers. Alkyd compounds often contain glass-fiber filler but may, for example, include clay, calcium carbonate, or alumina.
These unsaturated resins are produced through the reaction of an organic alcohol with an organic acid. The selection of suitable polyfunctional alcohols and acids permits selection of a large variation of repeating units. Formulating can provide resins that demonstrate a wide range of characteristics involving flexibility, heat resistance, chemical resistance, and electrical properties.
Diallyl Phthalates (Allyls)
Diallyl phthalates, or allyls, are among the best of the thermosetting plastics with respect to high insulation resistance and low electrical losses, which are maintained up to 400.degree. F. or higher, and in the presence of high humidity environments. Also, diallyl phthalate resins are easily molded and fabricated.
There are several chemical variations of diallyl phthalate resins, but the two most commonly used are diallyl phthalate (DAP) and diallyl isophthalate (DAIP). The primary application difference is that DAIP will withstand somewhat higher temperatures than will DAP.
DAPs are extremely stable, having very low after-shrinkage, on the order of 0.1 percent. The ultimate in electrical properties is obtained by the use of the synthetic-fiber fillers. However, these materials are expensive, have high mold shrinkage, and have a strong, flexible flash that is extremely difficult to remove from the parts.
Epoxies
Epoxy resins are characterized by the epoxide group (oxirane rings). The most widely used resins are diglycidyl ethers of bisphenol A. These are made by reacting epichlorohydrin with bisphenol A in the presence of an alkaline catalyst. By controlling operating conditions and varying the ratio of epichlorohydrin to bisphenol A, products of different molecular weights can be made.
Another class of epoxy resins is the novolacs, particularly the epoxy cresols and the epoxy phenol novolacs.
These are produced by reacting a novolac resin, usually formed by the reaction of o-cresol or phenol and formaldehyde with epichlorohydrin. These highly functional materials are particularly recommended for transfer-molding powders, electrical laminates, and parts where superior thermal properties, high resistance to solvents and chemicals, and high reactivity with hardeners are needed.
Another group of epoxy resins, the cycloaliphatics, is particularly important when superior arc-track and weathering resistance are necessary requirements. A distinguishing feature of cycloaliphatic resins is the location of the epoxy group(s) on a ring structure rather than on, the aliphatic chain. Cycloaliphatics can be produced by the peracetic epoxidation of cyclic olefins and by the condensation of an acid such as tetrahydrophthalic anhydride with epichlorohydrin, followed by dehydrohalogenation.
Epoxy resins must be cured with cross-linking agents (hardeners) or catalysts to develop desirable properties. The epoxy and hydroxyl groups are the reaction sites through which cross-linking occurs. Useful agents include amines, anhydrides, aldehyde condensation products, and Lewis acid catalysts. Careful selection of the proper curing agent is required to achieve a balance of application properties and initial handling characteristics.
Aliphatic amine curing agents produce a resin-curing agent mixture which has a relatively short working life, but which cures at room temperature or at low baking temperatures in relatively short time. Resins cured with aliphatic amines usually develop the highest exothermic temperatures during the curing reaction; thus the amount of material which can be cured at one time is limited because of possible cracking, crazing, or even charring of the resin system if too large a mass is mixed and cured. Also, physical and electrical properties of epoxy resins cured with aliphatic amines tend to degrade as the operating temperature increases. Epoxies cured with aliphatic amines find their greatest usefulness where small masses can be used, where room-temperature curing is desirable, and where the operating temperature required is below 100.degree. C.
Epoxies cured with aromatic amines have a considerably longer working life than do those cured with aliphatic amines, but they require curing at 100.degree. C. or higher. Resins cured with aromatic amines can operate at a temperature considerably above the temperature necessary for those cured with aliphatic amines. However, aromatic amines are not so easy to work with as aliphatic amines, because of the solid nature of the curing agents and that some (such as metaphenylene diamine) sublime when heated, causing stains and residue deposition.
Catalytic curing agents also have longer working lives than the aliphatic amine materials, and like the aromatic amines, catalytic curing agents normally require curing of the epoxy system at 100.degree. C. or above. Resins cured with these systems have good high-temperature properties as compared with epoxies cured with aliphatic amines. With some of the catalytic curing agents, the exothermic reaction becomes high as the mass of the resin mixture increases.
Acid anhydride curing agents are particularly important for epoxy resins, especially the liquid anhydrides. The high-temperature properties of resin systems cured with these materials are better than those of resin systems cured with aromatic amines. Some anhydride-cured epoxy-resin systems retain most electrical properties to 150.degree. C. and higher, and are not affected physically, even after prolonged heat aging at 200.degree. C. In addition, the liquid anhydrides are extremely easy to work with in that they blend easily with the resins and reduce the viscosity of the resin system. Also, the working life of the liquid acid anhydride systems is comparable with that of mixtures of aliphatic amine and resin, and odors are slight. Amine promoters such as benzyl dimethylamine (BDMA) or DMP-30 are used to promote the curing of mixtures of acid anhydride and epoxy resin.
Epoxies are among the most versatile and most widely used plastics in the electronics field. This is primarily because of the wide variety of formulations possible, and the ease with which these formulations can be made and utilized with minimal equipment requirements. Formulations range from flexible to rigid in the cured state, and from thin liquids to thick pastes and molding powders in the uncured state. Conversion from uncured to cured state is made by use of hardeners or heat, or both. The largest application of epoxies is in embedding applications (potting, casting, encapsulating, and impregnating) in molded parts, and in laminated constructions such as metal-clad laminates for printed circuits and unclad laminates for various types of insulating and terminal boards. Molded parts have excellent dimensional stability.
Melamines and Ureas (Aminos)
As compared with alkyds, diallyl phthalates, and epoxies, which are polymers created by addition reactions and hence have no reaction byproducts, melamines and ureas (also commonly referred to as aminos) are polymers which are formed by condensation reactions and do give off by-products.
Another example of this type of reaction is the polymerization reaction, which produces phenolics. Melamines and ureas are a reaction product of formaldehyde with amino compounds containing NH.sub.2 groups. Hence they are often also referred to a melamine formaldehydes and urea formaldehydes.
Amino resins have found applications in the fields of industrial and decorative laminating, adhesives, protective coatings, textile treatment, paper manufacture, and molding compounds. Their clarity permits products to be fabricated in virtually any color. Finished products having an amino-resin surface exhibit excellent resistance to moisture, greases, oils, and solvents; are tasteless and odorless; are self-extinguishing;
offer excellent electrical properties; and resist scratching and marring. The melamine resins offer better chemical, heat, and moisture resistance than do the ureas.
Amino molding compounds can be fabricated by economical molding methods. They are hard, rigid, and abrasion-resistant, and they have high resistance to deformation under load. These materials can be exposed to subzero temperatures without embrittlement. Under tropical conditions, the melamines do not support fungus growth.
Amino materials are self-extinguishing and have excellent electrical insulation characteristics. They are unaffected by common organic solvents, greases and oils, and weak acids and alkalies. Melamines are superior to ureas in resistance to acids, alkalies, heat, and boiling water, and are preferred for applications involving cycling between wet and dry conditions or rough handling. Aminos do not impart taste or odor to foods.
Addition of alpha cellulose filler, the most commonly used filler for aminos, produces an unlimited range of light-stable colors and high degrees of translucency. Colors are obtained without sacrifice of basic material properties.
Shrinkage characteristics with cellulose filler are a major problem.
Melamines and ureas provide excellent heat insulation;
temperatures up to the destruction point will not cause parts to lose their shape. Amino resins exhibit relatively high mold shrinkage, and also shrink on aging. Cracks develop in urea moldings subjected to severe cycling between dry and wet conditions. Prolonged exposure to high temperature affects the color of both urea and melamine products.
A loss of certain strength characteristics also occurs when amino moldings are subjected to prolonged elevated temperatures. Some electrical characteristics are also adversely affected; the arc resistance of some industrial types, however, remains unaffected after exposure at 500.degree. F.
Ureas are unsuitable for outdoor exposure. Melamines experience little degradation in electrical or physical properties after outdoor exposure, but color changes may occur.
Phenolics
Like melamines and ureas, phenolic resin precursors are formed by a condensation reaction. Phenolics are among the oldest, best-known general-purpose molding materials. They are also among the lowest in cost and the easiest to mold. An extremely large number of phenolic materials are available, based on the many resin and filler combinations, and they can be classified in many ways. One common way of classifying them is by type of application or grade. In addition to molding materials, phenolics are used to bond friction materials for automotive brake linings, clutch parts, and transmission bands. They serve as binders for wood-particle board used in building panels and core material for furniture, as the water-resistant adhesive for exterior-grade plywood, and as the bonding agent for converting both organic and inorganic fibers into acoustical- and thermal insulation pads, batts, or cushioning for home, industrial, and automotive applications. They are used to impregnate paper for electrical or decorative laminates and as special additives to tackify, plasticize, reinforce, or harden a variety of elastomers.
Although it is possible to obtain various molding grades of phenolics for various applications, as discussed, phenolics, generally speaking, are not equivalent to diallyl phthalates and epoxies in resistance to humidity and retention of electrical properties in extreme environments. Phenolics are, however, quite adequate for a large percentage of electrical applications. Grades have been developed which yield considerable improvements in humid environments and at higher temperatures. The glass-filled, heat-resistant grades are outstanding in thermal stability up to 400.degree. F. and higher, with some being useful up to 500.degree. F. Shrinkage in heat aging varies over a fairly wide range, depending on the filler used.
Polybutadienes
Polybutadiene polymers that vary in 1,2 microstructure from 60 to 90 percent offer potential as moldings, laminating resins, coatings, and cast liquid and formed-sheet products. These materials, being essentially pure hydrocarbon, have outstanding electrical and thermal stability properties.
Polybutadienes are cured by peroxide catalysts, which produce carbon-to carbon bonds at the double bonds in the vinyl groups. The final product is 100 percent hydrocarbon except where the starting polymer is the --OH or --COOH terminated variety. The nature of the resultant product may be more readily understood if the structure is regarded as polyethylene with a cross-link at every other carbon in the main chain.
Use of the high-temperature peroxides maximizes the opportunity for thermoplastic-like processing, because even the higher-molecular-weight forms become quite fluid at temperatures well below the cure temperature. Compounds can be injection-molded in an in-line machine with a thermoplastic screw.
Polyesters (Thermosetting)
Unsaturated, thermosetting polyesters are produced by addition polymerization reactions. Polyester resins can be formulated to have a range of physical properties from brittle and hard to tough and resistant to soft and flexible. Viscosities at room temperature may range from 50 to more than 25,000 centipoise (cP). Polyesters can be used to fabricate a myriad of products by many techniques, including but not limited to, open-mold casting, hand lay-up, spray-up, vacuum-bag molding, matched-metal-die molding, filament winding, pultrusion, encapsulation, centrifugal casting, and injection molding.
By the appropriate choice of ingredients, particularly to form the linear polyester resin, special properties can be imparted. Fire retardance can be achieved through the use of one or more of the following: chlorendic anhydride, aluminum trihydrite, tetrabromophthalic anhydride, tetrachlorophthalic anhydride, dibromoneopentyl glycol, and chlorostyrene. Chemical resistance is obtained by using neopentyl glycol, isophthalic acid, hydrogenated bisphenol A, and trimethyl pentanediol. Weathering resistance can be enhanced by the use of neopentyl glycol and methyl methacrylate. Appropriate thermoplastic polymers can be added to reduce or eliminate shrinkage during curing and thereby minimize one of the disadvantages historically inherent in polyester systems.
Thermosetting polyesters are widely used for moldings, laminated or reinforced structures, surface gel coatings, liquid castings, furniture products, fiberglass parts, and structures such as boats, including but not limited to sailboats, motor boats, and fishing boats; other motor vehicles such as automobiles, trains, motorcycles, trucks, and airplanes; gliders, sleds, and bathroom and kitchen components. Cast products include furniture, bowling balls, simulated marble, gaskets for vitrified-clay sewer pipe, pistol grips, pearlescent shirt buttons, and implosion barriers for television tubes.
By lay-up and spray-up techniques large- and short-run items are fabricated. Examples include boats of all kinds, such as pleasure sailboats and powered yachts, commercial fishing boats and shrimp trawlers, small military vessels, dune buggies, all-terrain vehicles, custom auto bodies, truck cabs, horse trailers, motor homes, housing modules, concrete forms, and playground equipment.
Molding is also performed with premix compounds, which are dough-like materials generally prepared by the molder shortly before they are to be molded by combining the premix constituents in a sigma-blade mixer or similar equipment. Premix, using conventional polyester resins, is used to mold automotive-heater housings and air-conditioner components. Low-shrinkage resin systems permit the fabrication of exterior automotive components such as fender extensions, lamp housings, hood scoops, and trim rails.
Wet molding of glass mats or preforms is used to fabricate such items as snack-table tops, food trays, tote boxes, and stackable chairs. Corrugated and flat paneling for room dividers, roofing and siding, awnings, skylights, fences, and the like is a very important outlet for polyesters.
Pultrusion techniques are used to make fishing-rod stock and profiles from which slatted benches and ladders can be fabricated. Chemical storage tanks are made by filament winding.
Silicones
Silicones are a family of unique synthetic polymers, which are partly organic and partly inorganic. They have a quartzlike polymer structure, being made up of alternating silicon and oxygen atoms rather than the carbon-to-carbon backbone, which is a characteristic of the organic polymers. Silicones have outstanding thermal stability.
Typically, the silicon atoms will have one or more organic side groups attached to them, generally phenyl (C.sub.6 H.sub.5 --), methyl (CH.sub.3 --), or vinyl (CH.sub.2 .dbd.CH--) units. Other alkyd aryl, and reactive organic groups on the silicon atom are also possible. These groups impart characteristics such as solvent resistance, lubricity and compatibility, and reactivity with organic chemicals and polymers.
Silicone polymers may be filled or unfilled, depending on properties desired and application. They can be cured by several mechanisms, either at room temperature [by room-temperature vulcanization (RTV)] or at elevated temperatures. Their final form may be fluid, gel, elastomeric, or rigid.
Some of the properties which distinguish silicone polymers from their organic counterparts are (1) relatively uniform properties over a wide temperature range, (2) low surface tension, (3) high degree of slip or lubricity, (4) excellent release properties, (5) extreme water repellency, (6) excellent electrical properties over a wide range of temperatures and frequencies, (7) inertness and compatibility, both physiologically and in electronic applications, (8) chemical inertness, and (9) weather resistance.
Flexible two-part, solvent-free silicone resins are available in filled and unfilled forms. Their viscosities range from 3000 cP to viscous thixotropic fluids of greater than 50,000 cP. The polymer base for these resins is primarily dimethylpolysiloxane. Some vinyl and hydrogen groups attached to silicon are also present as part of the polymer.
These products are cured at room or slightly elevated temperatures. During cure there is little if any exotherm, and there are no by-products from the cure. The flexible resins have Shore A hardness values of 0 to 60 and Bashore resiliencies of 0 to 80. Flexibility can be retained from -55.degree. C. or lower to 250.degree. C. or higher.
Flexible resins find extensive use in electrical and electronic applications where stable dielectric properties and resistance to harsh environments are important. They are also used in many industries to make rubber molds and patterns.
Rigid silicone resins exist as solvent solutions or as solvent-free solids. The most significant uses of these resins are as paint intermediates to upgrade thermal and weathering characteristics of organic coatings, as electrical varnishes, glass tape, and circuit-board coatings.
Glass cloth, asbestos, and mica laminates are prepared with silicone resins for a variety of electrical applications. Laminated parts can be molded under high or low pressures, vacuum-bag-molded, or filament-wound.
Thermosetting molding compounds made with silicone resins as the binder are finding wide application in the electronic industry as encapsulants for semiconductor devices. Inertness toward devices, stable electrical and thermal properties, and self-extinguishing characteristics are important reasons for their use.
Similar molding compounds, containing refractory fillers, can be molded on conventional thermoset equipment. Molded parts are then fired to yield a ceramic article. High-impact, long-glass-fiber-filled molding compounds are also available for use in high-temperature structural applications.
In general, silicone resins and composites made with silicone resins exhibit outstanding long-term thermal stabilities at temperatures approaching 300.degree. C., and excellent moisture resistance and electrical properties.
All of the conventional plastics shrink and/or crack to some degree when molded into large objects. To avoid these problems, elaborate curing schemes often have to be implemented which, in some cases, takes time and specialized equipment. What is needed is an additive or additives that will inhibit cracking and shrinkage and allow the rapid casting of large objects from a variety of prior art resins. What is also needed are additives that will strengthen objects made from conventional and gel coat resins without significantly increasing their weight.